A lot of people I know don’t want to pay $500 up front for a phone, but I think there is a lot of path dependency in this. It’s a lot more common in Europe to pay $500 or even more for a smartphone. When I lived in Denmark I did that at least twice. Americans are willing to pay $500 or more for an iPad. A non-trivial number paid $600 for the first generation iPhone, although not enough for Apple to continue with that model in the US.
Even if iPads or other tablets become subsidized by carriers, my guess is that a lot of tablets will still be sold without subsidy simply because people are used to it.
Another effect of being able to simply swap SIM cards is that there will be a larger population of useful old unlocked/unlockable phones out there with expired contracts. A lot of early GSM competition in Europe was driven by smaller carriers or MVNOs who could simply send you a SIM card to put in your old phone.